I 'd say it was because it's one of the most joyous, life-affirming films I've ever seen. It just makes me feel so good.
All the characters are engaging and funny. Woody is hilarious as the neurotic hypochondriac television producer who gets the idea he's got a brain tumor, and is almost as upset when he finds out he doesn't have one as he'd be if he did (have a brain tumor, that is.) He realizes that even if he is not going to die in the near future, he is going to die sometime, as are we all. He becomes obsessed with this idea, that death waits for us all, and if there's no God, no afterlife, what's the point of it all? So he embarks upon a quest to find Religion, a religion, any religion, that will satisfy him that there's something beyond human mortality.
Of course there's no answer to this, but Woody's desperate odyssey to find some meaning to a life that inevitably ends in death, some kind of certainty, is both something we can all relate to (maybe without the desperation) and extremely funny.
We don't find out till nearly the end of the movie how he resolves this. But there's no magic answer, no guru telling him some cosmic secret. Woody's epiphany is much more simple than that; it's that he discovers that life is sweet, and even if we only go around once and it all comes to an end, let's savor it while we're here. There's so much to savor. I can't express this the way Woody's character does in the film, it's best if you just watch the movie and vicariously experience his joy in this revelation.
There are lots of other delights in this film to enjoy along the way. All the actors are first-rate. Max von Sydow is especially moving as the rejected lover of Lee, one of the three sisters the movie follows over a period of two years. Lee is charmingly played by Barbara Hershey, while Mia Farrow as the "settled" sister, captures the two sides of Hannah, as someone who's both almost annoyingly perfect (at least as perceived by others) yet is actually as needy and vulnerable as everyone else.
But the most engaging character in Hannah and Her Sisters has got to be Holly, the quirky "off-beat" slightly edgy sister. Dianne Wiest won a well-deserved Oscar for this role. She makes Holly funny, touching, and sympathetic.
